The human spine consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments. There are 24 movable vertebrae—7 cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, and 5 lumbar (back) vertebrae. Each vertebra has a somewhat cylindrical bony body (centrum), a number of winglike projections (processes), and a bony arch. The arches are positioned so that the space they enclose forms the vertebral canal. The vertebral canal houses and protects the spinal cord, and within it the spinal fluid circulates. Ligaments and muscles are attached to various projections of the vertebrae. The bodies of the vertebrae form the supporting column of the skeleton. Fused vertebra make up the sacrum and coccyx, the very bottom of the vertebral column.
The spine is subject to abnormal curvature, injury, infections, tumor formation, arthritic disorders, and puncture or slippage of the cartilage disks. Modem spine surgery often involves the use of spinal stabilization/fixation procedures such as a vertebral body fusion procedure to correct or treat various acute or chronic spine disorders and/or to support the spine. In conjunction with these procedures, some spinal implants may be utilized to help stabilize the spine, correct deformities of the spine such as spondylolisthesis or pseudarthrosis, facilitate fusion, or treat spinal fractures. Some spinal implants such as a spinal fixation system may provide fused and/or rigid support for the affected regions of the spine. For example, a spinal fixation system may include a corrective spinal implant that is attached to selected vertebrae of the spine by screws, hooks, and clamps. The corrective spinal implant may include spinal rods or plates that are generally parallel to the patient's back. The corrective spinal implant may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems can be used to correct problems in the cervical, thoracic, and lumbar portions of the spine, and are often installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process. Spinal fixation systems when implanted inhibit movement in the affected regions in virtually all directions.
More recently, so called “dynamic” systems have been introduced. Dynamic spinal stabilization systems can better match a patient's anatomy than some spinal stabilization systems used to provide static support. When implanted in a patient, a dynamic spinal stabilization system can allow at least some movement (e.g., flexion, extension, lateral bending, or torsional rotation) of the affected regions of the spine in at least some of the directions, giving the patient a greater range of motion. Dynamic stabilization systems can be used in scenarios in which vertebral body fusion is not desired, in which vertebral body (re)alignment is desired, and in which it is desired to support or strengthen degraded, diseased, damaged, or otherwise weakened portions of the spine.
Often, spinal stabilization systems include rods which can bear a portion of the forces that would otherwise be transmitted along the spine. These rods may be implanted in pairs or in other numbers along portions of the spine of interest. Some spinal stabilization systems may support a portion of the spine including only two vertebrae (and associated anatomical structures) while some spinal stabilization systems support portions of the spine extending beyond two vertebrae. Spinal stabilizations systems can be used to support various portions of the spine, including the lumbar portion of the spine and the thoracic portion of the spine. Regardless of the number of rods implanted, or the portion of the spine in which they may be implanted, the rods can be attached to one or more vertebrae of the spine to provide support and stabilize, align, or otherwise treat the region of the spine of interest. Surgical personnel may use one or more anchor systems to attach the rods to one or more vertebrae. One such anchor system includes pedicle screws constructs which define slots, keyways, grooves, apertures, or other features for accepting and retaining stabilization rods which may be static, dynamic, or a combination of both. In many pedicle screw constructs, pedicle screws are placed in vertebrae selected by surgical personnel.